Free piston pneumatic hammer



Feb. 9, 1937.

3 sheets-sheet 1 'Filed .April 18, 1935 ehm um.

nu@ ww und um MM. QNWNQN KNRM Feb. 9, 1937'. s. oHLssoN FREE PISTON PNEUMATIC HAMMER 4 Filed April 18, 1955 3 Sheets-Sheet 2 '|NVENTOR` M @ww/MW Feb. 9, 1937. s. oHLssoN 2,070,552 E FREE P I S TON PNEUMATI C HAMMER Filed April 18. 1935 3 Sheets-Sheet 3 ll-7132? 7. 57 29 'w/ /m A i L i 37a t 37 @awww/ 72:

-5- 22,54 fm (14 f2? INVENTOR Patented Feb. 9, 1937 UNITED sTATs PAT-ENT OFFICE Samuel Ohlsson, Wilkinsburg, Pa., assignor to Robert A. Bundle, Edgewood, Pa.

Application April 18, 1935, Serial No. l17,001

13 Claims.

This invention relates to free plunger hammers and has for an object the production vof such a hammer which eliminates the necessity of employing expensive and elaborate auxiliary equipment such as is essential in connection with the use of a modern pneumatic hammer.

A further object is to produce a hammer of the pneumatic free plunger type wherein the free plunger responds to variations in fluid pressure which are occasioned by mechanisms located within and forming a unitary part of the hammer.

A further object is to produce a hammer of the class de'ned which is comparable, lfrom the standpoint of utility and effectiveness, with present day pneumatic hammers, but which is cheaper to build, install for operation, and operate than such hammers.

A further object is to produce a new and improved procedure or method of operating the impact plunger of a hammer having the functional characteristics of the modern free plunger pneumatic hammer.

'I'hese and other objects which will be made more apparent bythe further description of my invention are attained by means of a hammer embodying the features and characteristics disclosed by the accompanying drawings.

In the drawings:

Figure l is a longitudinal sectional view of a hammer embodying my invention;

Fig. 2 is a sectional View along the horizontal plane 2 2 of Fig. l;

Figs. 3-8 inclusive are diagrammatic views disclosing various positions occupied by the principal moving parts of the hammer during one complete cycle of operation; and

Fig. 9 is a more or less diagrammatic view of a hammer embodying my invention and illustrates a modification of structural details which may be employed in connection therewith.

The hammer illustrated in Figs. 1 and 2 of the drawings as an embodiment of my invention includes an impact plunger It which is located within a cylinder II formed within the barrel I2 of the hammer and which has many of the characteristics of the free piston of the modern pneumatic hammer and responds to variations in uid pressure at its rear end. The plunger I reciprocates within the cylinder I I and on its forward stroke is projected at a high Velocity against an impact tool or bit I3 of such a tool. The variations in the fluid pressure within the cylinder I I, at the rear end of the plunger I8, are occasioned by a power driven piston I4 which is located within the rear end of the cylinder I I.

The arrangement of the apparatus is such that during each cycle of operation air is trapped within the cylinder between the forward `end of the piston I4 andthe yrear end of the plunger I!! and is vslightly compressed vby the forwardly moving piston I4 and to a degree necessary to overcome the friction of rest and the inertia 4of the plunger I0. As soon as the plunger I0 'starts vto move forward, i. e., in the direction of the arrow of Fig. 2, its resistance to motion is materially decreased because the friction of motion is materially less than the friction of rest, and consequently the plunger will, in responding to the pressure of the trapped air behind it, accelerate in velocity at substantially the same rate as the piston I4. rIhus it is apparent that the plunger I9 is operatively coupled to the piston I4 by what may .be termed a variable air coupling and, While the plunger accelerates in speed at a rate substantially equal to the rate of acceleration of the piston during the rst portion of the piston stroke, the deceleration of the piston I4-during the latter portion of its stroke-does not affect the speed of the plunger I0 as it moves toward the impact -tool preparatory to delivering a blow to that tool. As will be more fully described, the plunger I0 completes its impact stroke at substantially the maximum velocity of the piston I4, with the result that, during normal operations, it is projected against the tool bit at a projectilelike velocity.

A characterizing feature of my Iinvention is that air is rst trapped within the cylinder II and then discharged from the cylinder during the forward movement vof the piston I4, with the result that vthe piston completes a portion of its forward stroke without doing any work other than that required to expel previously trapped' air from the cylinder II. The return stroke of the piston I4 is alsoy characterized bythe fact that the space within the cylinder II, between the rear end of the plunger and the forward end of the piston, is first cut off from communication with the atmosphere and thenopen to the atmosphere. Under such conditions the returning piston I4 tends to create a vacuum or a substantial rarefication of the trapped air as it starts on its return stroke, with the result that the plunger Ill, responding to the preponderating pressure of the atmosphere on `its forward end, follows the piston throughout a substantial portion of the pistons return travel; and then communication between the atmosphere and the interior of the cylinder is again established bythe piston uncovering an air port, and the 'piston f completes'its return stroke without doing any work other than drawing a full charge of air intorthe cylinder preparatory to a repetition of the cycle here described.

Referring more specifically to the drawings, the barrel I2 of the hammer, which encloses the cylinder II and the piston I4 and which also forms a guide for the reciprocations of the plunger I9, may be said to constitute the body portion of the hammer. As shown in Figs. 1 and 2, a breech portion I5 is secured to the barrel by means of a ring nut I6, which engages a ilange El formed near the rear end of the barrel and operates to draw a threaded annular flange I8 of the breech portion, against the rear face of the iiange Il and thus position the-breech with relation to the barrel. It will, of course, be understood that any suitable means for securing these parts together may be employed, since this feature forms no part o f my present invention, except insofar as it is essential as a partof an operative structure which may be manufactured ata reasonable cost. An inspection of Figs. 1 and 2 disclosesrthat the annular llange I8 surrounds and seats on a rearward extension of the barrel, which projects beyond the flange I7. This detail is desirable because it provides a cylinder having an internal bore free from joints `and seams; it, however, is'not essential.-

The breech portion I5 encloses and constitutes a casing for the operating mechanism of the piston I4. In the hammer illustrated, a power driven crank shaft I9 is enclosed by and is mounted in suitable bearings provided within the breech portion. The crank pin is operatively connected to the piston I4 by means of a piston rod I 2I and the crank shaft is provided with suitable balancing discs 22y for counterbalancing the weight of the crank pin and the associated portion of the piston rod. Y

In Fig. 2 of the drawings, the crank shaft I9 is shown operatively coupled to a motor driven flexible shaft 23, which as is usual, is enclosed by a ilexible casing 24. This particular feature of the disclosure does -not constitute an impor- L'tant part of my invention,vsince it will vbe undercrank shaft I9 to a driving motor. I, however,

prefer to employ an electric motor as the source of power and it will be apparent that various positional relations between the shaft I9 and the driving motor may be employed.

Communication between the interior of the cylinder, or the portion thereof intermediate the plunger I El and the piston I4, is controlled by the plunger ID and piston I4 operating in conjunction with a valve 25 which moves synchronously with the crank shaft I9. In Fig. 2, the valve 25 is shown as a sleeve valve which, in effect, constitutes an extension of the crank shaft I9 Yand is journaled in a sleeve bearing 25a. The bearing 25a, is supported by and enclosed in the breech portion I5 and constitutes a ported casing for the valve 25. The valve 25 is provided with two ports 28 and 29 (see Fig. 3) which cooperate with a port 26 formed within the sleeve bearing 25a, for the purpose of alternately establishing and cutting off communication between a passage 21 and the interior of the sleeve valve 25. The interior of the sleeve valve communicates directly with the atmosphere through ports formed in a cap nut 30 secured to the breech portion and forming an end enclosure for the sleeve valve. As shown in the diagrammatic views of Figs, 3 to 8, the ports 28 and 29 of the sleeve valve 25 are located in substantially diametrically opposed positions, with relation to the center of rotation of the sleeve valve, and the port 28 is substantially larger than the port 29, i. e., its peripheral length is substantially greater than that of the port 29.

The diagrammatic views above referred to also disclose the relationship of the ports 28 and`29 with the port 26 during the operation of the hammer. munication with the port 26 and communicates with the interior of cylinder II through two spaced branch passages 21u. and 2lb. Both the piston I4 and the plunger I9 have a tight sliding fit with thecwall of the cylinder I I and, with the arrangement of the branch passages illustrated, the plunger Ill and the piston I4 cooperate with the valve 25 in controlling communication between the interior of the cylinder and the atmosphere. A manually operated valveSImay also be employed for controlling a communication between the portion of the cylinder II intermediate the plunger and the piston and the atmosphere. This valve operates as a throttle valve and its function is to establish communication between the interior of the cylinder II and the atmosphere independently of the position of the portsV 28 and 29 of the valve 25 with relation to the port 26.

As shown in Fig. 1, valve 3I consists of a slide valve located in a passage suitably formed within the breech portion I5 and it is adapted to contro-l communication between a port 32, which is always open to the atmosphere, and a port 33, which, by 'means of a passage 33', communicates with the passage 2l (Fig. 2). the valve 3l is located is suitably countersunk to provide a shoulder which constitutes a seat for a coil spring 34 surrounding the valve 3| and acting between the seat and a shoulder formed by a valve extension 3l of increased diameter. The spring acts to hold the valve 3l open, i. e., in a. position establishing direct communication 'between the ports 32 and 33, in which position the extension 3i projects beyond the confines of the valve enclosing bore and is engaged by a valve actuating trigger 35 pivotally mounted on the breech portion I5. From this it will be apparent that the spring 34 not only acts to hold the valve in the so-called open position, but also acts to hold the extension 3| against the trigger. As will hereinafter be more fully described, when the valve 3I is in the position illustrated in Fig. 1, the passage 2l is always in free communication The passage 21 is always in open com-- 'Ihe bore in which with the atmosphere, consequently the reciprocations of the piston I 4 will not control the movements of the plunger I9.

The diagrammatic views of Figs. 3 to 8 disclose Various positions occupied by the parts at difierent intervals during a complete cycle of operation. In describing the operation illustrated by cycle the plunger I9 is in its rearmost position 75 andan annular shoulder 36 formed on the plunger near its forward end, is in engagement with an annular shoulder 36a formed on the interior surface of'the barrel I2 by enlarging the bore at the forward end of the barrel. As the crank pin 29 continues its movement in the direction of the arrow of Fig. 3, the piston I 4 is advanced on what may be termed its forward or compression stroke and its closes the branch passage 2lb, thus shutting oif the space S from the passage 27, because in this position of the piston the plunger I is either just starting to move forward or is still in its rearmost position, but in any event, it is in such a position as to close the branch passage 21a. This position of the parts is illustrated in Fig. 4.

During this portion of the cycle the advancing piston Hi is accelerating in speed and the acceleration increases until the piston rod 2i reaches a position substantially as disclosed in Fig. 5, wherein the line defined by the center o-f the crank pin and the center of the piston pin 3'! is tangent to the circle defined by the center of the crank pin in its rotation. During this portion of the cycle the plunger I9, moving in response to the pneumatic pressure within the space S occasioned by the advancing piston I, has reached the end of its stroke or has been projected at a high velocity against the tool I3.

From the diagrammatic views and also from the previous description it will be apparent that the maximum pressure which can be developed within the space S by the advancing piston M is only that which is necessary to overcome the friction of the plunger l@ and the inertia eiect occasioned by the mass of the plunger. When, however, the piston i4 is in the position illustrated by Fig. 5, the rear end of the plunger has uncovered the branch passage 21a and has thus established communication between the space S and the passage 2l. In this position the port 28 of the valve 25 is just starting to uncover the port 26, or in other words, the parts are all in the positions such that a further movement will establish free communication between the space S and the atmosphere. As the piston I4 continues its forward movement or as the crank pin 20 moves from the position illustrated in Fig. 5 to the position illustrated in Fig. 6, the port 28 remains in register with the port 25, due to the peripheral length of the port 28. From this it is apparent that as the piston M advances from the position shown in Fig. 5 to that shown in Fig. 6, its only function is to expel the air previously trapped in the space S. However, as the piston arrives at the extreme end of its forward stroke the port 23 moves out of register with the port 26, thus again cutting off communication between the space S and the atmosphere. This relation of ports 2S and 28 is illustrated in Fig. 6.

The continuing movement of the crank pin starts the piston back, but this rearward movement of the piston tends to create a vacuum-or a partial vacuum-within the space S and the degree of the vacuum thus produced will depend upon the frictional and inertia forces to be overcome in moving the plunger I0 rearwardly. That is to say, the return or rearward movement of the plunger I0 is occasioned by the preponderance of atmospheric pressure--acting at the forward end of the plunger-over the sub-atmospheric pressure within the space S, consequently the direct effect of the piston I4 in moving the plunger rearwardly will depend upon the degreeV of vacuum, Within the space S, necessary to overcome the above mentioned forces.

Figure 7 diagrammatically illustrates the positions of the parts as the crank pin, moving in the direction of the arrow, moves the piston Iii through the accelerating portion of its rear or return stroke. There the port 26 is closed by the valve and in addition the branch passages 21a and 2lb are both closed by the parts operating within the cylinder I I. As a result of this relationship of the parts, the so-called variable air coupling tends to, and does function to move the plunger IE' with the piston I4.

Figure 8 illustrates positions of the parts such that the rearwardly moving piston I 4 is. ju'st starting to open the branch passage 21h and the port 29 is in register with the port 26 and is moving to establish full and free communication between the'passage 21 and the atmosphere; It is, therefore, apparent that a further movement of the piston I4 toward the rear end of its stroke, will establish full andl free communication between the space S and the atmosphere. Fig. 8 also discloses thatin the portion. ofV the cycle thereillustrated, the rearward movement of the plunger I0 has been stopped by the engagement of the shoulder 36 with the shoulder 36a and that consequently, throughout some. portion of the pistonsv rearward movement, the space S has been enlarged and the degree of vacuum existing therein increasedby this enlargement. As soon as the passage 2lb isuncoveredby the piston, atmospheric pressure will again be established in the space S. As the parts movefrom the positions shown, in Fig. 8, to, the positions shown in Fig. 3, the cycle of operation is completed, but during this period, the space S is retained in open communication with the atmosphere through the branch passage 2lb, the passage21, the port 26,

the port 29, the interior of thevalve 25 andthe ports of the cap nut 30. As previously described, as the parts move to the positions illustratedin Fig. 3, the port 29. moves out of register withthe port 26, thus trapping a charge of air within the space S and the communicating passagesy L formed within the hammer and the breech por.- tion thereof.

A comparison of Figures 3 and 6 discloses .that the travel of the piston M is considerablygreater than that of the plunger I0. In the illustrated embodiment the throw of the plunger is approximately half thatv of the piston, the stroke of the plunger being limited by the tool I3 and by the shoulders 36 and 36a. This difference in stroke or throw of the plunger and piston is an important feature of my improved hammer, since it permits the plunger to lag behind the piston on the return stroke without destroying the necessary synchronous operation of the two elements. This permissible lag makes it possible to reduce the speed of the return stroke of the plungeri. e., the plungers movement to its rearmost position-and thus reduce the impact between the shoulders Sii-36a. In other words, the'fact that the plungers backward travel isv substantially less than (approximately half of) the pistons backward travel makes it possible for the plunger to travel much more slowly on its backward stroke than the average speed of the piston, consequently, the plunger on coming to rest in its rearmost or initial position, will not impose excessive jars and shocks on the hammer, on the operators hand and the operating parts of the hammer. On the other hand, the short travel of the plunger.

permits it to arrive at :its.initialpositionxinzplentyi 7'5 toolV bit I3.

An analysis of the cycle of operation as illustrated by Figures 3-8, discloses that the operation of the plunger |73 closely approximates the operation of the free piston in the modern pneumatic hammer. As the piston I4 of the hammer illustrated, movesthroug'h the accelerating portion of its forward stroke, it starts to compress' theair trapped as above described. As it closes the passage 2lb, this compressing action von the air .thus trapped 'within the space Sis more effective, but invany event, theV degree of pneumaticpressure built up .Within the space S will` not substantially exceed that necessary to overcome the friction of rest and the inertia of the plunger. As soon as the -plunger starts its forward movement, the frictional resistance is reduced, because the'friction of motion is less than the friction of rest, consequently, the plunger III will move forward with at least the same acceleration as the pistonId and it is apparent that the expansionv effect of the compressed air may impart a greater acceleration to the plunger'l l] than is imparted to the piston I4 by the crank pin as the crank pin moves from the position shown in Fig. 3 to that shown in Fig. 5. In any event, the arrangement of parts is such that the advancing plunger Ill will deliver its blow to the tool I 3 while it is moving under conditions established within theworkingpassages of the hammer by the piston o I4 asY that piston moves forwardly at its greatest speed. f

It will also be apparent that the force of the blow delivered by the plunger In will depend upon the mass of the plunger and the final velocity of the plunger. It will also be apparent that the final velocity of the plunger is directly dependent.Y upon the maximumV velocity of the piston I4 as it traverses its forward stroke. It will therefore be apparent that the force of each blow delivered by the hammer may be varied by varying the number of reciprocations, per unit of time, of the piston I4, and may be increased by speeding up the motor driving the-shaft I9. Thus. the force of each blow and the power output of the hammer, per unit of time, may be varied by varying the speed of the driving motor.

In Figure 2, and in eachY of the diagrammatic views, Figs. 3-8, I have shown an auxiliary passage 3'I which is adapted to communicate with a porty 31a 'formed in the cylindrical wall-of the piston I4 and thus establish communication between the space S and the atmosphere under a condition which may be encountered at the time of starting the hammer. In this connection, it will be' understood that the rear of the piston I4 is alwaysY in direct communicationY with the Yatmosphere through the valve 25 as is shown in Fig. 2. If, at the time of starting, the plunger I i) isY at the forward end of its stroke or in the position illustrated inFigs. 5 and 6 and the piston I4 is at the rear end of its stroke or in 'the'poy sition illustrated in Figs. 3 and 8, an excessive volume of air will be trapped within the hammer cylinder by the forwardly moving piston, with the result that an excessive pressure will be produced unless some means is employed for establishingV normal operative conditions within the hammer cylinder. If we, therefore, assume that atst'arting, the piston and plunger occupy the positions above defined, itwill be apparent thatY the forwardly moving piston will compress the trapped air `and that the plunger movingv in response to any excess pressure which may be Vthus produced, will place the passage' 3l in direct' communication with the space S so that the excess pressureV thus developed is exhausted to the atmosphere when the port 31a of the piston reg-` isters with the passage 31'..V

This forward movement of the piston is only resisted by the Y,tool I3 which is looselyheld in place, consequently, unless the tool is applied to the work with unusual force,normal conditions are immediately establishedwithinv the cylinder and normal operation then follows.

VThe first operation of starting the hammer is of course the'starting of the drivingmotor and where the hammer is provided with a throttle valve, such as the valve 3|, which is normally held open, the reciprocations of the piston Ii will have no effect on the plunger except possiblyV the momentary effect of moving it against the tool I3, since the passage 2l is in open communication with the. atmosphere and the functioning of the valve therefore has no effect in trapping air within the hammer. apparent that even though the passage 2l is maintained in open communication with the at-V mosphere by the open throttle valve 3I, the advancing piston I4 can only move into metal to metal contact with the plunger I0 at the extremev end of its forward stroke and then only `when the plunger is in such a forward position as to It will, however, be Y uncover the branch passage 21a since in all other some extent, by the throttle valve and in this way the force of each blow struck by the plunger Ill can be controlled, to some extent. Here again, the difference in plunger and piston travel is of importance. It will be apparent that variations in the position of the throttle valve 3| will occasion variations in the maximum `air pressure produced within the space S by the forwardly moving piston id and that consequently, where Y the throttle is partially open, the plunger moving in response to the pressure in space S may, in effect, lag behind the piston and thus strike a blow considerably lighter than under normal operating conditions, where the valve 3l is fully closed. From the foregoing it is apparent that the valve 3i is in effect a by-pass valve, since it bypasses atmospheric pressure around the valve 25 into the passage 2 andthus into the space S of the cylinder I2. f

In Fig. 9, I have shown a further modification of the apparatus in whichsomev structural details are varied. This gure corresponds generally to Fig. 2 except the plunger limiting shoulder is located at the rear instead of at the front of the plunger. The plunger is provided with a rearward extension Ia which forms a shoulder 36h near therear end of the plunger and this shoulder cooperates with a shoulder formed within the hammer cylinder by an annular inset 40 whichA may be made of special steel. Inother respects, the Vstructural. details are substantially as shown in Fig-2.` Y

In Fig. 9, and also in Fig. 2, I have shown a passage 4lwhich` establishes direct communication between the external atmosphere and the interior vof the hammer in front of the plunger IU. It will, of course, be apparent that one or more of such passages may be employed and that they are desirable because the more or less loose fit around. the tool bit E3 m-ay not be adequate toy maintain atmospheric air pressure ahead of the plunger l0 at all times during operation.

From the foregoing, it will be apparent that my invention makes it'possible to obtain all the advantages of the modern free piston pneumatic hammer without the necessity of employing the expensive equipment such as air compressors, air tanks, air lines, air valves, etc. which are necessary parts of the modern air hammer equipment. It will also be apparent that hammers embodying my invention are just as convenient for use as the modern air hammer, since, while it is necessary to employ a flexible shaft to drive the hammer-where the driving motor is not a unitary part of the hammeror to employ an electrical lead-in and outlet-where the driving motor is a unitary part of the hammernevertheless, these connections are less cumbersome than the ordinary air hose made necessary by the use of the usual types of pneumatic hammers. In addition, even where a separate motor is employed, the flexible shaft need not be as long as the usual air hose, since the motor equipment is relatively small and therefore can be assembled. a part of a portable unit, and may be placed adjacent to the work in hand.

While I have described but one method of procedure in connection with the illustrated apparatus, it will be apparent that the separate steps of this method may be varied and also that the structural details of the apparatus illustrated may be varied without departing from the spirit and scope of my invention as defined by the appended claims.

What I claim as new and desired to secure by Letters Patent is:

l. In combination in a pneumatic hammer, a cylinder, a free plunger and a piston operating within said cylinder, a crank shaft, means connecting said shaft to said piston, driving means for said shaft, a valve operating synchronously with said piston, a piston controlled passage and a plunger controlled passage between said cylinder and said valve, said valve operating in conjunction with said piston and plunger to alternately establish and cut 01T communication between said cylinder and the atmosphere once during each reciprocation of said piston.

2. In combination in a pneumatic hammer, a cylinder, a free plunger and a piston located in spaced relation within said cylinder, means for positivelyY actuating said piston, a valve operating in synchronism with said piston actuating means, piston and plunger-controlled passages communicating with the passage of said valve, means for limiting the stroke of such plunger to a stroke substantially-less than that of said piston, means in addition to said valve for controlling communication between the atmosphere and the interior of said piston, and adjustable means for controlling said l-ast mentioned means.

3. In combination in a pneumatic hammer, a cylinder, a piston and a free plunger located in spaced relation within said cylinder, means for positively reciprocating said piston, means including a valve and piston and plunger-controlledE passages for controlling communication between the atmosphere andv the cylinder space betweensaid piston and plunger, and means for operat-f ing said valve in synchronisml with said piston.-

4. In combination in a pneumatic hammer, 'a cylinder, a piston and a free plunger located in spaced relation within said cylinder, means for positively reciprocating said piston, means including a valve and piston and plunger-controlled passages for controlling communicationr between the atmosphere and the cylinder space between said pistonr and plunger, means for operating said valve in synchronism with said pis-f ton, and means for positively limiting the stroke of said plunger to substantially half the stroke of said piston.

5. In combination in a pneumatic hammer, a cylinder, a piston'and a freeplunger located in spaced relation within said cylinder, means for positively reciprocating said piston, means 'including a valve and piston and plunger-controlled passages for controlling communie-ation between the atmosphere and the cylinder space between said piston and plunger, means for operating said valve in synchronism with said piston, means for positively limiting the stroke of said plunger to substantially half the stroke of said piston, and adjustable means for delivering atmospheric air to said cylinder around said -valve during the operation of said valve.

6. In combination in a pneumatic hammer, a cylinder, a piston and a plunger located in spaced rel-ation within said cylinder, means for positively reciprocating said piston, means including a valve, and plunger and piston-controlled passages for establishing and cutting of communication between the atmosphere and the cylinder space between said piston and plunger during each reciprocation of said piston, means for actuating said Valve in synchronism with the reciprocations of said piston, and means for bypassing said valve.

7. In a pneumatic hammer, a cylinder, a free plunger and a piston operatively mounted within said cylinder, means for positively reciprocating said piston within said cylinder, a passageway communicating with said cylinder at spaced positions therealong, and a rotary valve operably connected to said piston and movable therewith, said valve intervening between said passageway and the atmosphere and controlling communication therebetween.

8. In a pneumatic hammer, a cylinder, a piston and a free plunger located within said cylinder, an atmospheric vent passage for establishing communication between the atmosphere and the space between said piston and plunger within such cylinder, a valve for controlling such communication, and means for positively actuating said valve and said piston in timed relation with each other.

9. In a pneumatic hammer, a cylinder, a piston and a free plunger located in spaced relation within said cylinder, an atmospheric vent passage communicating with said cylinder at spaced points therealong for establishing communication between the atmosphere and the space between said piston and cylinder at intervals during the movements of said piston, a valve for controlling such communication and means for positively actuating said valve in timed relation with the operation of said piston.

10. In a pneumatic hammer, a cylinder, a piston and a free plunger located within said cylinder, an atmospheric vent passage communicating with said cylinder for establishing communication between the atmosphere and the space within said cylinder between said piston and plunger at intervals during the operation ot such parts, a valve for controlling such communication, driven means for reciprocating said piston and means for positively actuating said valve in timed relation with the movement of said piston.

v11. In a pneumatic hammer, a cylinder, a piston and a free plunger located within said cy]- inder, an atmospheric vent passage for establish! ing direct communication between the atmosphere and the space within saidl cylinder between said piston and plunger at intervals dur-A ing the operation of said piston and plunger, a

valve for controlling such communication, driven means for reciprocating said piston, means for positively actuating said valve in timed relation with the reciprocation of said piston, and a manually controlled valve for establishing communication between the atmosphere and said passage independently of the operation of said first mentioned valve.

12. In a pneumatic hammer, a cylinder, a piston and free plunger located within said cyl'in-k der, an atmospheric vent passage having a piston controlled port communicating with the interior of said cylinder for establishing direct communication between the atmosphere and the space within said cylinder between said Vpiston and plunger, a valve for establishing and cutting 01T communication between said passage and the atmosphere, driven means for reciprocating said piston and means for positively actuating said valve in timed relation with the movement of said piston.

13. In a pneumatic hammer, a cylinder,'a piston, a free plunger located 'within said cylinder, an atmospheric vent passage having piston and plunger controlled ports communicating with the space within said cylinder between said piston and plunger, a valve for establishing and cutting oi communication between said passage and the atmosphere, driven means for reciprocating said piston, and means actuated thereby for positively actuating said valve in timed relation with the movement of said piston. 

